1. Field of the Invention
The present invention relates to improvement of low temperature fluidity of fuel oils, and more particularly to chemical treatment of fuel oils to improve their low temperature fluidity.
2. Description of the Prior Art
Upon encountering low temperatures, fuel oils tend to develop fluidity problems. In particular, paraffins in the fuel agglomerate at low temperatures to form a waxy semi-solid or gel-like material that plugs pipes and filters, inhibiting transmission of the fuel to, for example, an engine.
Conventionally, this problem is treated by adding to the fuel a chemical composition called a low temperature fluidity modifier. The low temperature fluidity modifier can co-crystalize with or adsorb the paraffins in the fuel oil to precipitate the paraffin before agglomeration or to modify paraffin crystal growth so that the resulting irregularity in size and shape of the crystals inhibits agglomeration or efficient packing of the crystals, thereby reducing the tendency toward plug formation. By contrast, pour point depressants are directed simply to viscosity reduction of fluids. Thus, while studies have shown a relation between low temperature fluidity of a fuel and the pour point or cloud point of the fuel, the mechanism of low temperature fluidity modifier operation and the problem to which low temperature fluidity modifiers are directed differ significantly from those of pour point depressants. Therefore, despite the apparent relationship between low temperature fluidity and pour point, they typically require different treatments.
Because low temperature modifiers operate by affecting the crystal growth of the paraffins in the fuel being treated, the selection and composition of a low temperature fluidity modifier for a particular fuel is based on the nature of the paraffins in that fuel. For example, low temperature modifiers typically are coordinated with the paraffins in the fuel so that the solubility characteristics of the modifier added to the fuel match the solubility characteristics of the paraffins in the fuel. Thus, if a fuel contains C.sub.20-24 paraffins that crystallize at 10.degree. F., the modifier is typically designed to crystallize at about 10.degree. F. as well, thereby to interfere with the crystallization of the paraffins. Accordingly, it is well known to those of ordinary skill in the art of low temperature fluidity modification to select and to adjust the array of aliphatic chain lengths to balance overall solubility based on the paraffin content of the fuel to cause the additive to precipitate out of the fuel at the desired temperature. In fact, it is common to produce esterified olefin/maleic anhydride copolymers for use in low temperature fluidity modifier additive compositions by esterifying certain olefin/maleic anhydride copolymers with an array of aliphatic alcohols having chain lengths in the range of from about four to about forty carbon atoms, and to select the distribution of aliphatic chain lengths in that range in coordination with the paraffins in the fuel as discussed above.
Despite the existence of a variety of low temperature fluidity modifiers, none provides completely satisfactory performance in all fuels. In fact, because of the disparities in the characteristics of fuel oils, particular low temperature fluidity modifiers meet with varying success from fuel to fuel. Thus, there is a continual search for ever more effective low temperature fluidity modifiers, particularly for use in various fuels.